Some dielectric studies / by Brian James Cooke

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Abstract

Two methods of approach are current in the literature
for the interpretation of dielectric relaxation. One is that
due to Debye which assumes that the relaxation process has its
origin in the retardation of molecular reorientation due to
frictional forces acting on the molecule. The other treats
dipole rotation as a rate process in which the dipole must
acquire a certain amount of energy in order to surmount a
barrier separating two equilibrium positions of orientation.
The dielectric relaxation times of some large ketones have
been determined at four temperatures using a cell which does not
appear to have been used up to this time for measuring the
dielectric constant and loss of low loss liquids. The molecules
measured were selected because of their size and shape, five
were ellipsoidal and one was disc-like. For the ellipsoidal
molecules^ the position of the dipole within the molecule was
varied to investigate its effect on the relaxation time. A
number of equations, based on the Debye model, which attempt to,
account for the size of molecular relaxation time are examined.
It is found that only the Fischer equation is satisfactory in
predicting the effects of dipole direction within the molecule.
The experimentally measured activation energies for all
the large molecules were found to be similar and only a little
higher than those observed for smaller molecules. In an attempt
to understand these values a model is proposed based on the energy expended by the molecule during its reorientation process.
The approach leads to a method for predicting the effect of
solvent on dielectric relaxation time. It is found that the
relaxation time depends exponentially on the internal pressure
of the medium surrounding the relaxing species, and the
activation energy can be accounted for in terms of the product
of an activation volume and the internal pressure. From the
activation volume an estimate is obtained of the angle through
which the dipole rotates. For small molecules it is found that
the angle is of the order of 20 degrees which indicates a fairly
large jump accompanying the reorientation. For the larger
molecules, however, the angle is much smaller, hence, the behaviour
resembles Brownian rotational diffusion.